In Yii, we can define an action class and ask the controller to instantiate it. And we always create ourselves actions class in rootPath/protected/extensions, and the view file we called in the actions class always are located in rootPath/protected/views/ControllerNAME or rootPath/themes/ThemeNAME/ControllerNAME, then when we use the same actions class in different controllers, we have to copy it from one controller to the other controller, so we also need the "same" view file work for us, here is an example.
Most applications will have one or two sidebars and often you want to control the content that should appear in the sidebar based on the action. For example you will want to show something different on the homepage as opposed to some view page. While achieving this you generally also want to avoid putting the layout into each view, as that would mean if you wanted to change the layout (e.g. put the sidebar on the left instead of the right or do some funky layout which requires an additional div tag to be added) you'd have to go through every view, which does not achieve good code re-use. It is also bad practice to have lots of if statements in column2 layout just so that you can generate the sidebar correctly.
Occasionally one wishes to set systemwide parameters for an application, such as a contact address for email, an application name, or setting an option that guides major behavior. Yii provides for setting of static parameters in the configuration file, and this article talks about how to do it conveniently.